Line circuits for key telephone systems are well known based on the Bell System 400B line card (U.S. Pat. No. 3,239,410 to Morse et al.) and the 400D card (U.S. Pat. No. 3,436,488 to Barbato et al.).
Later reference patents such as No. 3,647,983 to Fitzsimons et al. and 3,715,516 to Ebrahimi show variations using three relays to produce the same results as the later Bell patent using the same input conditions and producing the same outputs. These two patents use a line relay, a ring control relay and a combined busy and hold control relay in essentially the same manner as shown by Barbato. In the former two patents, the two-winding line relay is held energized in all in-use conditions. In the Barbato patent the two-winding line relay is energized only during ring condition and hold condition and is restored during the busy or seize condition. The Fitzsimons reference employs transistors and diodes as the operative members and Ebrahimi uses AND gates and OR gates to control the operation of the two control relays.
In all the references noted above, the line relay is a two-winding relay which has a single contact pair in the circuit of the control relays while the single-winding control relays have plural contacts in the lamp leads, the ringing supply in the circuit to the line relay and in a line bridge to provide the hold. All the references noted employ the conventional A lead which is grounded during the seize and which lead is open circuited on the transition to hold prior to opening of the line loop through the line conductors.